Automatic field strength control for precipitators



July 5, 1960 L. L. LITTLE 2,943,697

AUTOMATIC FIELD STRENGTH CONTROL FOR PRECIPITATORS Filed July 22, 1957 fi- POW-5E 3/01/0581: 7ZAf/SFOEM- EB mvo Sauecz z Pencroz PEcr/F/EIe /0 l 'J /6 I2 Powse SPARK Mae/van: 37 Cueezur Hun/H52 F I 1. r E 2 lurzcenj -35- Fi e. 1.

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[MW/P) L. LITTLE INVENTOR.

BY aw ATTORNEY 5 United States Patent AUTOMATIC FIELD STRENGTH CONTROL FOR PRECIPITATORS Larry L. Little, Los Angeles, Calif., assignor, by mesne assignments, to Joy Manufacturing Company, a corporation of Pennsylvania Filed July 22, 1957, Ser. No. 673,528

14 Claims. (Cl. 183-7) The present invention relates generally to electrical precipitators of the type used for the purpose of collecting or precipitating particles suspended in a gas stream by means of a high voltage electric field maintained between opposing electrodes. More particularly, the present invention relates to means for automatically controlling the strength of the electrical field in such precipitators in a manner which maximizes precipitator operating efiiciency.

Basically, an electrical precipitator comprises one or more pairs of opposing electrodes between which a high voltage electric field is maintained, and through which a gas containing particles to be precipitated is passed. It is customary practice to ground the collecting electrode of each pair whilethe other electrode (the discharge electrode) is connected directly to a source of high voltage electrical power. As the voltage applied across an electrode pair is raised from zero, a voltage level is reached at which there is corona discharge at the discharge electrode; and this condition is accompanied by a small but definite electric current flow between the two electrodes, this cur-rent commonly being referred to as the corona current. The value of the corona current depends on the applied voltage, the spacing between electrodes, and the nature of the gas and particles between the electrodes.

If the voltage applied between opposing electrodes is raised beyond the level where corona discharge begins while holding the other factors constant, the corona current increases; and there is eventually reached a voltage level at which intermittant sparking occurs between the electrodes. Since the individual sparks each represent a current flow between the electrodes, this sparking adds to the total flow of current between the opposing electrodes. The total discharge current flowing between the electrodes is thus composed of a fraction caused by corona discharge and another fraction caused by sparks. In general each spark represents a substantial increase in the instantaneous value of the discharge current flowing between the electrodes; but since each spark is of short duration, a single spark has but a comparatively small efiect in increasing the average value of the total discharge current.

As the voltage applied between the electrodes is raised beyond the level where sparking begins, corona current flow increases, and sparking takes place between the electrodes at an increasing rate and with greater intensity until eventually conditions between the electrodes become unstable, and a dielectric breakdown or arc-over occurs. When an arc-over occurs between the two electrodes, the voltage falls rapidly to a small fraction of its previous value and the total discharge current between the electrodes rises substantially, but the collection elficiency drops to a low value.

Corona current is, in general, a measure of the ability of the precipitator to charge suspended particles to be collected and, together with the voltage, determines collection efficiency. Therefore, it is desirable to operate the precipitator at the highest practical values of corona 2,943,697 H Patented July 5, 19 0 current and voltage without creating the conditions that bring about arcing with its attendant drop in corona and voltage. Consequently, at the normal optimum operating conditions there is a considerable amount of intermittent sparking current in addition to a relatively high corona current. The portion of the total discharge current caused by corona discharge is comparatively uniform for any given voltage level, but the amount of spark current and the arc-over voltage level may change quite rapidly due to several factors: changes in the character of the gasses passing through the precipitator, changes in the concentration and electrical characteristics of the suspended material in the gas stream, changes in the amount of material collected upon the electrodes, and various other conditions. Also, it should be appreciated that the quantity of electricity contributed to the total discharge current by each spark is not constant, since some of the sparks may be longer or of greater intensity than others, and so pass a larger quantity of electricity. The length and intensity of each spark also varies with the factors discussed above.

Thus the problem of maintaining optimum operating conditions in a precipitator is considered best solved by constantly maintaining the electrode voltages at the highest level which will not cause arc-over; however, any practical solution is complicated by the fact that the areover voltage level is frequently changing in a given installation, some times changing rapidly and some times rather slowly.. .Since the voltage at which arc-over occurs has no constant value, the precipitator can not be set to operate at optimum conditions by a fixed control. Variable controls for precipitators have been devised which operate by the principle of counting the total number of sparks for a given period of time, then controlling the electrode voltage to maintain a certain preset number of sparks per minute. However, since the strength and duration of sparks vary according to the same factors which cause a variation in arc-over voltage level, these devices have only a limited usefulness in applications where the arc-over level varies rapidly. Also, it has been found that the optimum number of sparks per minute in any given installation with a relatively stable arc-over voltage level might vary from 50 to several hundred sparks per minute, and is best determined by actual test under the particular conditions involved for each different installation.

Thus it becomes a general object of my invention to provide a device for automatically controlling the field strength between the electrodes to maintain optimum operating conditions within the precipitator and automatically to change the applied voltage level in response to a change in operating conditions.

It is also an object of my invention to provide a device of this character which is responsive to the strength, duration, and number of sparks per unit time occurring between electrodes in such a manner that the voltage between electrodes is raised or lowered automatically to maintain some preset value of average spark current.

An additional object of my invention is to provide a device operative in this manner responsive to the integral of spark current over some fixed period of time, wherein the period of integration and the preset value of average spark current are both adjustable by simple controls.

A further object of my invention is to provide an auto These objectsand advantages of my invention are obtained by providing means for detecting changes in the field strength between the electrodes, means for 'sepa-.

rating from the total changes those variations due to a amass-r change in spark strength, duration or frequency, and means for changing the voltage applied to the electrodes such that changes in spark, strength, duration or frequency are counteracted by changes in. applied voltage to. maintain a relatively constant average: spark. current. In general, changes.- in the field strength between. the electrodes. are reflected as. changes in. the total discharge current between the two electrodes, and hence as changes in the. current input, to the electrodes. Therefore, in. my invention I detect changes inthe field, strength by placing an. inductance, in the form of the load winding of a saturable reactor, in series with the power input tothe electrodes- Changes in the. discharge current are. thus detected as. changes. in the voltage. drop. across the load winding of the saturable reactor.

However, not all of the voltage variations appearing across. the. saturable.- reactor load winding are caused by variations in the spark current. Variations are also caused by the. A.-C. power frequency applied, and by changes in corona current. The efiects. of. the AC. power input. are removed by placing. a. rectifier andpower ripple frequency filter across. the saturable reactor load winding. The effects of steady state. corona currentand other steady state factors are removed by placing. in the filter a D.-C. blocking capacitor in series with the output of the power frequency filter. The ouput from the rectifier and filter is created. by transient current changes; i.e. sparks, which. are. characteristically of a very short. time duration, and transient changes in. the corona current, which are usually of a very long time duration compared to sparks.

An integrating circuit is provided at the output side of the blockin capacitor to integrate the said output current flow due to individual sparks over a fixed period. of time, which is controllable, and which is normally set at around l5-20 seconds. The efiect of transientchanges. in corona current in this integrating circuit is. negligible, due to their long time duration with respect to the time. constants of the circuit. Thus the current level in. the integrating circuit indicates the average spark. current flow during any period of time equal to the fixed integrating period.

The. voltage level present in the. integrating. circuit is amplified to a suitable operating level in an. amplifier, and then applied back to the control winding of the saturable reactor. The current flow through the. control winding deter-mines. the impedance of the saturable. reactor; and since the saturable reactor load winding is in series with the power source, the impedance of the saturable reactor determines. the amount of voltage which is applied to the electrodes.

In summary, my invention detects variations in electrode discharge current. by a saturable. reactor in series with the power source and the electrodes, then separates from the total variations the change caused by a fluctuation in. average spark current to. produce a D.-C. signal which is amplified before using it to' control the: impedance. of the. saturable reactor in such a manner that the.

voltage applied to the electrodes is changed to counter act the variation in average spark current. The field strength between the electrodes is thus maintained. at; its optimum value under rapidly varyingconditions; A particular advantage of my invention is that this corrective action is relatively independent of the particular conditions involvedin any given precipitator installation, so that no. extensive testing is required to adapt the control for use in difiering installations.

How the above objects and advantages of my invention, as well as. others not specifically mentioned herein, are attainedwill be better understood by reference to the following description and to the annexed. drawing, in which:

Fig. 1 is a block diagram of my invention as used with a precipitator.

e 2 is a s h mati dia ram. o anart cula embod ment of my invention as used with a precipitator.

Referring particularly to Fig. 1, the electrical precipitator is shown schematically as comprising collecting electrode 10 and discharge electrode 11, which oppose each other. The collecting electrode is normally grounded, as shown, while the discharge electrode is maintained at a relatively high voltage by means; of an A.-C. power source applied between the two' electrodes through a rectifier. The voltage between. the: two' electrodes forms. an. electrostatic. held at 12. My invention is shown. as including the saturable reactor placed between thez.A.-.C. power source, and the rectifier, with an output voltage going from the reactor to the power rectifier and filter, and thence to the. spark current integrator circuit, and then through an amplifier back to the control winding of the saturable reactor.

Fig. 2 shows a schematic diagram of. a particular embodiment of my invention as used with an electrical pres cipitator. Referring. particularly to Fig; 2, the precipie tator rectifier 6. is formed by four semi-conductor diodes 7 which. are connected to. operate: as a full wave. bridge rectifier, as is well. known in the art. Voltage step-up transformer 3 is. placed between the bridge rectifier 6 and the A.-C. voltage. source, with secondary Winding, 5.

connected across the input to the bridge rectifier and with primary winding 4 energized. by an A.-.C. voltage applied from power. input. terminals 1 and. 2. The alternating current source, is ordinarily a commercial, source providing power at a commercial voltage, usually 220 volts or 440 volts, and at a commerciallfrequency, usually 60. cycles per second.

Connected'in series with the primary winding of power transformer 3 is saturable reactor 13, which acts as both. the detecting and control element of my invention. Variations in the strength. of field 12. are detected asvariations in the. voltage drop across the load winding. 14 of the saturable reactor; and in response to these. voltage. varitions the current flow through the control. winding. 15 of the saturable reactor is suitably altered by the actionv of my invention. This in turn. alters the impedance: of load winding 14,, and thuschanges the amount of. voltage appearing across. transformer primary winding 4, and therefore the voltage applied between. opposing, electrodes. 10 and 11-.

Load winding 14 is connected by conductors 16 to the input terminals of a filter-integrator circuit which has. at its input end a bridge rectifier formed. by four semiconductor diodes. 17. The rectifier has common input.

terminals with this filter-integrator circuit. One. output. terminal of the bridge rectifier is connected to a circuit common lead 18, and the other output terminal is connected in parallel to a power frequency filter, and. to a resistor 32 which is the discharge path for capacitors. 20., 2-2 and 23'. In addition resistor 32 has a function explained later in connection with the. circuit of transistor 30. The power frequency filter is comprised of induct.- ance 21' and capacitors 20" and. 22 connected in a pi configuration. The filter acts to remove power frequency ripple from the rectified output of load winding14. Thus the output of the pi filter contains only steady state D..-C..,. and transients caused by variations in. discharge current.

The steady state D'.-C. component is filtered out by D;-C. blocking capacitor 23, which also acts. toreduce the effect of any transients due to changes in corona current. The. corona current. transients, which are of at relatively IOJW frequency, are. attenuated. by capacitor 23;

while the. spark currenttransients,.which. are of a relative-- To. the: right. ofblocking: capacitor and. receiving the outputztherefrom; is the; porn tion of the circuit that functions as a filter-integrator. This portion comprises semi-conductor diodes 24 and 25 connected in parallel to the output side of capacitor 23, storage capacitor 26, and a resistance preferably separated into fixed resistor 27 and variable resistor 28.

The negative portion of the spark transients is clipped by semi-conductor diode 24, which acts as a short circuit to ground for any negative voltages. Semi-conductor diode 25 acts as an open circuit to positive voltages. The positive portion of the spark transients is passed through diode 25 to integrating capacitor 26 where it causes a positive charge to be developed on capacitor 26 in accord with the intensity and duration of the spark. When the spark ends, diode 25 opens, and the charge thus accumulated on storage capacitor 25 discharges through serially connected resistors 2'7 and 28 and the input of transistor 30. Within a given range, the discharge time constant of the circuit is controllable by variable resistance 28. In a typical application it is usually set for a fixed discharge period of around to seconds. Since a typical spark lasts only second, it can be seen that the discharge current through resistors 2(7 and 28 represents the total effect of many sparks over the selected period of about 15-20 seconds. Since the amount of charge delivered to capacitor 26 depends on the strength, duration and frequency of sparks during this period of time, it can be seen that the discharge current through resistors 27 and 28 is a function of the average value of the total spark current between the electrodes during that period of time.

The output of the spark current integrating circuit is amplified by transistor 30, which utilizes a. portion of the steady state D.-C. current available from the bridge rectifier as its source of power and bias. The current available through resistor 32. is always greater than the current required by the transistor. Voltage regulator diode 3-1 serves to by-pass the excess current and maintains a. constant voltage source for bias and power to transistor 30. The transistor bias is applied through resistor 29 to the transistor base. Power to be controlled by transistor 30 is developed across voltage regulator diode 31. The negative side of voltage regulator diode 31 is connected directly to the emitter of transistor 30. The positive side of the regulator diode is connected through the signal winding of amplifier 35 and inductance 34 to the collector of transistor 30.

The output of transistor 30 is applied through a filter circuit and conductors 36 to an amplifier 35. Though other types of amplifiers may be used, it is preferred to use a magnetic amplifier at 35. The filter consists of inductance 34 and capacitor 33' connected in an L configuration. The filter acts to prevent ripple frequency feedback from the amplifier 35 to the transistor The amplifier in this particular embodiment is a magnetic amplifier which has a non-linear relationship between current input and current output. This non-linear characteristic is utilized as part of the invention, to reduce the etfect of spark current when the device is set to operate the precipitator at low levels of current. This non-linear characteristic of the magnetic amplifier is desirable because an increment of corona current at low current levels has a much greater effect on collection efficiency of the precipitator than the same increment of current at a high current level. if collection efficiency is plotted on the y-axis against current on the x-axis, the curve rises steeply at first and then flattens to almost a zero slope at high current values. The non-linear characteristic of the amplifier is chosen to make it much. more sensitive in the lower range of current values. 1

It should also be noted that magnetic amplifier 35 performs the function of inverting the direction of change of the DC. signal output from the integrator circuit. It has the characteristic that its output is a maximum when its input is a minimum, and vice versa. Hence, a decreasing incoming signal is inverted and becomes an increasing control current sent to reactor Winding 15. As a result of this inverse relation between input and output,

asides? reactor 13 is caused to counteract any change in field strength at precipitator 12 whereby the field strength is maintained at some established value, or close thereto.

The R-C or time constant of the filter circuit is chosen to discount the effect of a momentary shower of sparks which does not reflect a real change in operating conditions; for example, a shower of sparks caused by a cloud of heavy particles falling between the electrodes. These sparks come at very short intervals of time and may occur at the rate of several per second. Such sparks do not represent a real change in operating conditions and it is better if the circuit can partly ignore them, otherwise the shower would drive the precipitator voltage to a very low level. When these frequent light sparks occur, their effect is damped by capacitor 23 if the interval between successive sparks is less than the discharge time of the capacitor, which typically might be about A second. Sparks coming oftener than that prevent the capacitor from fully discharging and so do not cause as much increase in output through diode 25 as if each spark charged capacitor 23 from a fully discharged state.

The output of amplifier 3-5 is connected by conductors 37 to the control winding 15 of the saturable reactor 13, where it determines the impedance of the reactor, and thus the proportion of the applied source voltage that is absorbed across the load winding A drop in the current through the control winding increases the impedance of the saturable reactor, thus increasing the amount of the supply voltage dropped across the load winding, and consequently decreasing the voltage applied to the primary winding of the power transformer 3. This in turn lowers the voltage applied between the electrodes 19 and 11. A rise in the current through the control winding decreasesthe impedance of the saturable reactor, thus decreasing the amount of supply voltage dropped across the load winding, and consequently increasing the voltage applied between the electrodes.

Having described the construction of my invention, the operation is briefly as follows:

In the absence of sparking between the precipitator electrodes, the storage capacitor 26 discharges slowly at .a rate determined by the decay characteristics of the discharge circuit. Discharge of the condenser creates a D.-C. current signal which is amplified by the transistor and then applied to the magnetic amplifier; and under the assumed conditions this signal is at a minimum. Since the output of the magnetic amplifier is at a maximum when input to it from the filter-integrator circuit is a. minimum, the magnetic amplifier now exerts the maximum correcting efiect upon the saturable reactor. The result is that the saturable reactor operates to raise the voltage applied to maintain the field between the electrodes and thus increases the field strength between the electrodes. When conditions in the precipitator become such that the applied voltage approaches the arcover level, sparking is fairly intense and frequent and there is a marked increase in the average sparking current flowing between the electrodes. This increase in sparking current is detected according to my invention by an increase in the voltage drop across the load winding of the saturable reactor. This increase in voltage drop creates an increased output current which is applied to the filter-integnator circuit where the output is rectified and all components are filtered out except a portion of the current which bears a known relation to the current flowing between electrodes as a result of sparking. This transient fraction charges the storage capacitor; and thereby increases the discharge current from this same capacimagnetic amplifier,'after amplification by the transistor.

The output from the magnetic amplifier is now decreased by the rise in input and thus the saturable reactor is caused to reduce the voltage applied to maintain the field between the precipitator electrodes. The invention there- 7 fore. operates to maintain the field strength between electrodes at the most elficieut operating level despite fluctuations in operating conditions, thus obtaining maximum 'efii'ci'ency of collection of the particles suspended in the gas being treated;

From the foregoing description it will be understood that various modifications may be made in the control device without departing from the spirit and scope of my invention; and that the same methods of regulating the field strength may be employed without using exactly the same arrangement of elements illustrated herein. Accordingly, it is to be understood that the foregoing de scription is considered to be illustrative of, rather than limitative upon, the appended claims.

' I claim;

1. An automatic field. strength control. device for use in combination with. an electrical precipitator wherein an electrical field is maintained between opposing electrodes by means of an A.-C. power source connected through a transformer and rectifier to said opposing electrodes, said control device comprising: a saturable reactor having a control winding and a load winding, said load winding being connected in series with the primary winding of said transformer; a filter-integrator circuit with input terminals connected across said load winding, said filterintegrator circuit producing a D.-C. output signal corresponding to the integral of transient changes in the A.-C. voltage. drop across said load winding; and an amplifier with input terminals connected to the output terminals of said filter-integrator circuit, and with output terminals connected to the control winding of said saturable reactor; whereby the field strength between said opposing electrodes is. automatically maintained at some value related to the average spark current flowing between said opposing electrode-s independent of the total. corona cur-- rent flowing between said opposing electrodes.

. 2. An automatic field strength control device for use in combination with an electrical precipitator wherein an electrical field is maintained between opposing electroded by means of an AC. power source connected through a transformer and rectifier to said opposing electrodes, said control device comprising: a saturable reactor having a control winding and a load winding, said load winding being connected in series with the primary winding of said transformer; a filter-integrator circuit with input terminals connected across said load winding, said filter-integrator circuit producing a D.-C. output signal corresponding to the integral of transient changes in the A.,-C.v voltage drop across. said load winding; and an amplifier with input terminals connected to the output terminals. of said fiIter-integrator circuit, and with output terminals connected to the control winding of said saturable reactor; said filter-integrator circuit comprising a rectifier having input termials that are the input terminals of said filter-integrator circuit and output terminals, of which one is common, that are connected to a first filter circuit, said first filter circuit adapted to block the frequency of said A.-C. power source; a D.-C. booking capacitor connected to the non-common output terminal of said first filter circuit; an integrating circuit with input terminals connected between the output terminal of said D.-C. blocking capacitor and the common terminal of said first filter circuit; a second filter circuit connected between the output terminals of said integrating circuit, said second filter circuit adapted to block the frequency of said A.-C. power source; and. the output terminals of said second filter circuit comprising the output terminals of said filter-integrator circuit; whereby the field strength between said opposing electrodes is automatically maintained at some value related to the average spark current flowing between said opposing electrodes independent of the total corona current flowing between said opposing electrodes.

3., A device as defined in claim 2 wherein said rectifier 8 comprises diode units connected, to form a bridge rectifier circuit.

4. A device as definedv in claim 3 wherein said diode units are semi-conductors.

5. A device as defined in claim 2 wherein said first filter circuit comprises a first capacitor connected in parallel with the output terminals of said rectifier, an inductance connected in series with the noncommon output terminal of said rectifier, and a second capacitor connected between the output terminal of said inductance and the common terminal of said rectifier.

6. A device as defined in claim 2 wherein said second filter circuit comprises a capacitor connected in parallel with the output terminals of said integrating circuit and an inductance connected in series with the non-comrnon output terminal of said integratingv circuit.

7. A device as defined in claim. 2 wherein said first amplifier comprises a. magnetic amplifier having a nonlinear relationship between input current and output current, said non-linear relationship adapted to form a part of the action of said automatic field strength control device in maintaining said field strength at the appropriate level in relation to said average spark current between said opposing electrodes.

8.. A device as defined in claim 2 wherein said integrating circuit comprises an. integrating network with input terminals connected between the output terminal of said D.-C. blocking capacitor and the common output terminal of said first filter circuit, and a second amplifier with input terminals connected across the output terminals of said integrating network, and the output terminals. of, said second amplifier comprising. the output terminals of said integrating circuit.

9. A device as. defined in claim 8 wherein said integrating network comprises. a first diode unit connected between the output terminal of said D.-C. blocking capacitor and the common outputxterminal of said first filter circuit, with the anode of said first diode unit connected to the common terminal of said first filter circuit, a second diode unit having its anode connected to the junction formed by the output terminal of said D.-C. blocking capacitor with the cathode of said first diode unit, an integrating capacitor connected between the cathode of said second diode unit and the anode of saidfirst diode unit, and a variable resistance with one terminal connected to the junction formed by the cathode of said second diode unit with the non-common terminal of said integrating capacitor, such that the output terminals of said integrating network are formed by the unconnected terminal of said variable resistance and the common terminal of said integrating capacitance.

10. A device as defined in claim 9 wherein said first and second diode units are semi-conductors.

11. A device as defined in claim 8 wherein said second amplifier comprises a transistor, the base of said transistor being connected to the non-common output terminal of said integrating network and to a bias network, said bias network comprising a first resistor connected between said transistor base and the cathode of a third diode unit whose anode is connected to the common output terminal of said integrating network, the emitter of said transistor being connected to the common. output terminal of said integrating network, and the output terminals of said amplifier being formed by the collector of said transistor and the junction of the cathode of said third diode with said first resistor; and a second resistor connected in series with the third diode between common and non-common output terminals of the rectifier, said diode togetherv with current passing through the second resistor comprising a power source for current controlled by the transistor and a voltage source for the bias network.

12. A device as defined in claim 11 wherein said third diode unit is a semi-conductor.

13. An automatic field strength control device for use in combination with an electrical precipitator wherein an electrical field is maintained between opposing electrodes by power from an A.-C. source connected through a transformer and a rectifier to the precipitator, said control device comprising: a saturable reactor with its load winding connected through the transformer with the input terminals of the rectifier; means connected across said load winding for sensing a change in voltage drop across the load winding; means for producing a D.-C. signal representing a known function of said change in D.-C. voltage drop across the load winding; an amplifier electrically connected to said means for producing a D.-C. signal to receive said D.-C. signal and having its output connected to the control winding of said saturable reactor whereby the saturable reactor continuously changes the field strength in inverse relation to the change in strength of the D.-C. signal.

14. An automatic field strength control device for use in combination with an electrical precipitator wherein an electrical field is maintained between opposing electrodes by means of an A.-C. power source connected through a transformer and rectifier to said opposing electrodes, said control device comprising: a saturable reactor having a control winding and a load winding, said load winding being connected in series with the primary winding of said transformer; a filter-integrator circuit responsive to changes in the current flowing in said load winding to said transformer primary, said filter-integrator circuit producing a continuous D.-C. output signal corresponding to the integral of transient changes in the current flowing through said load winding; and an amplifier with input terminals connected to the output terminals of said filter-integrator circuit, and with output terminals connected to the control winding of said saturable reactor; said filter-integrator circuit comprising a rectifier having input terminals that are the input terminals of said filter-integrator circuit and output terminals, of which one is common, that are connected to a first filter circuit, said first filter circuit adapted to block the frequency of said A.-C. power source; a D.-C. blocking capacitor connected to the non-common output terminal of said first filter circuit; an integrating circuit with input terminals connected between the output terminal of said D.-C. blocking capacitor and the common terminal of said first filter circuit; a second filter circuit connected between the output terminals of said integrating circuit, said second filter circuit adapted to block the frequency of said A.-C. power source; and the output terminals of said second filter circuit comprising the output terminals of said filter-integrator circuit; whereby the field strength between said opposing electrodes is continuously maintained at some value related to the average spark current flowing between said opposing electrodes independent of the total corona current flowing between said opposing electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,771,150 Welts Nov. 20, 1956 2,798,571 Schaelchlin et al. July 9, 1957 2,823,757 Klemperer Feb. 18, 1958 

